1. Field of the Invention
The present invention relates to a system and method for enhanced combustion control in an internal combustion engine.
2. Description of Related Art
In a 4-stroke cycle gasoline injected internal combustion engine, when certain conditions are met within a charge of lean air/fuel mixture, auto-ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge. This results in very stable power output, very clean combustion and high thermal efficiency. NOx emission produced in auto-ignited combustion is extremely low in comparison with spark-ignited combustion. In spark-ignited combustion, the burnt gas temperature is highly heterogeneous within the charge with very high local temperature values, creating high NOx emission. By contrast, in controlled auto-ignited combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values, resulting in very low NOx emission.
In auto-ignited combustion, speed, at which pre-reaction of fuel proceeds with elevation of temperature and pressure, governs an initiation timing of auto-ignited combustion. Accordingly, operating conditions under which auto-ignited combustion may be performed are restricted to a limited engine speed and load range. Auto-ignited combustion, if performed under operation conditions outside of this limited range, may cause the engine to knock or suffer from unstable combustion. Rapid cylinder pressure elevation, if not too excessive to cause the engine to knock, may result in making an unacceptably high level of combustion noise.
In an attempt to govern an initiation timing of auto-ignited combustion, JP-A 10-196424 describes a control piston to provide additional compression around top dead center during compression stroke. A fuel injector is positioned to inject fuel into each intake port. Air/fuel mixture is created by the fuel injected and air introduced into each cylinder during induction stroke of each engine cycle. Increased temperature and pressure due to additional compression of the control piston causes ignition of the air/fuel mixture charge.
With this known technique, operating conditions under which auto-ignited combustion may be performed remain restricted to such engine speed and load range where the air/fuel mixture charge will not be ignited before TDC during compression stroke.
To maintain thermal efficiency, auto-ignited combustion must be initiated and completed within a limited duration of crank angles. This limited duration shrinks in time as engine speed increases. At high engine speeds, a pressure elevation per unit time needs to be increased to complete combustion within a reduced period of time in each engine cycle. At high or heavy loads, the pressure elevation per unit time increases due to an increase in the total amount of heat generated in each engine cycle. They cause the increased probability for engine to knock at high engine speeds and/or high loads, making it difficult to extend operating conditions, under which auto-ignited combustion may be performed, toward high engine speeds and/or high loads.
It is therefore an object of the present invention to provide a system and method for enhanced combustion control in an internal combustion engine to extend operating conditions, under which auto-ignited combustion may be performed, toward high engine speeds and/or high loads.
According to one aspect of the present invention, there is provided a system for enhanced combustion control in an internal combustion engine having at least one cylinder, a reciprocating piston within the cylinder to define a combustion chamber therein, comprising:
a fuel supply system having a fuel injector positioned to directly inject fuel into the combustion chamber, the fuel supply system being capable of performing a split injection wherein a first fuel injection in each engine cycle precedes a second fuel injection that occurs during compression stroke in the same engine cycle;
a spark plug positioned to produce a spark to ignite a first air/fuel mixture portion created due to the second fuel injection to initiate a first stage combustion,
the first stage combustion elevating temperature and pressure high enough to cause auto-ignition of a second air/fuel mixture portion surrounding the first air-fuel mixture portion to initiate an auto-ignited second stage combustion; and
an engine controller programmed to perform control over beginning time of the auto-ignited second stage combustion in response to at least one of the engine speed and engine load.
According to another aspect of the present invention, there is provided a method for enhanced combustion control in an internal combustion engine having at least one cylinder, a reciprocating piston within the cylinder to define a combustion chamber therein, comprising:
performing a split injection wherein a first fuel injection in each engine cycle precedes a second fuel injection that occurs during compression stroke in the same engine cycle;
producing a spark to ignite a first air/fuel mixture portion created due to the second fuel injection to initiate a first stage combustion, thereby to cause elevation of temperature and pressure high enough to cause auto-ignition of a second air/fuel mixture portion surrounding the first air-fuel mixture portion to initiate an auto-ignited second stage combustion;
determining beginning time of the auto-ignited second stage combustion in response to at least one of the engine speed and engine load; and
performing control over at most an injection timing of the first fuel injection, an injection timing of the second fuel injection, a spark timing, a proportion of fuel quantity of the second fuel injection to the total quantity injected in each engine cycle, and an exhaust gas recirculation (EGR) rate to cause the auto-ignited second stage combustion to begin at the determined beginning time.